1. Field of the Invention
This invention relates to an optical communications module having M laser diodes (LDs), M photodiodes (PDs) and M lightwaveguides (Mxe2x89xa71) suitable for bi-directional simultaneous optical communications which allocate a single fiber to both upward and downward signal streams. This invention also relates to an optical communications module having M laser diodes (LDs), M monitoring photodiodes and M lightwaveguides (Mxe2x89xa71) suitable for optical communications which allocate a fiber to an upward signal stream.
This application claims the priority of Japanese Patent Applications No. 2001-391143 filed on Dec. 25, 2001 and No. 2002-320705 filed on Nov. 5, 2002, which are incorporated herein by reference.
Bidirectional communications gives an optical fiber a role of carrying optical signals in both directions between a central station and a plurality of subscribers (ONU: optical network units). LD/PD modules are equipped at the ONUs and the station. An LD/PD module should separate light paths into a PD part and an LD part at ends of optical nets. Various separation elements have been proposed. Requirements for the LD/PD path separation elements are low-division loss, low optical crosstalk, low electrical crosstalk, and low electromagnetic crosstalk.
Optical crosstalk means that strong light emitted from an LD goes into a highly sensitive PD and induces noise in receiving signals. Simultaneous bidirectional communications system uses two different wavelengths xcex1 and xcex2. xcex1(1.3 xcexcm) is a upward signal wavelength which is sent from ONUs to a central station. xcex2(1.55 xcexcm) is a downward signal wavelength which is sent from the central station to the ONUs. Photodiodes (PDs) generally used in optical communications have an InGaAs light receiving layer (sensing layer or active layer) which has sensitivity between 1.0 xcexcm and 1.6 xcexcm. The InGaAs-PDs sense both xcex1 and xcex2. Thus, optical crosstalk from LDs to PDs should be eliminated. xe2x80x9cOpticalxe2x80x9d means that the medium of transmitting noise is light.
Another problem is electric crosstalk from LDs to PDs. Electric crosstalk means that strong driving currents for LDs mixe with weak photocurrents of PDs via a package or wirings. xe2x80x9cElectricxe2x80x9d means that the medium of transmitting noise is electric currents. A further problem is electromagnetic crosstalk. LDs generate electromagnetic waves which fly in space to the PDs and induce noise in the receiving signals. xe2x80x9cElectromagneticxe2x80x9d means that the medium of carrying noise is electromagnetic waves. Reduction of optical, electrical and electromagnetic crosstalk is ardently required for LD/PD modules in the optical communications.
2. Description of Related Art
There are some types of modules for allocating an LD and a PD. FIG. 12 shows a discrete type of an LD/PD module. The discrete type LD/PD module aligns an optical fiber 85 and an LD 86 along a straight beam line, positions a Wavelength Division Multiplexer (WDM) 87 slanting by 45 degrees to the beam line at a middle point in the straight beam line, and disposes a PD 88 in a vertical direction which crosses the beam line at right angles. In the module, transmitting light signals emitted from the LD 86 simply pass the WDM 87 and go into the optical fiber 85. Receiving light signals propagating in the optical fiber 85 are reflected at the WDM 87 and are guided into the PD 88 for generating photocurrents which represent receiving signals.
The WDM 87 is employed for selecting wavelengths in the module. The WDM, which is an optical device made by piling a plurality of sets of more than two kinds of transparent dielectric thin films having different refractive indices, allows a first wavelength (xcex1) to pass at a ratio of nearly 100% but reflects a second wavelength (xcex2) at a rate of nearly 100%. The WDM has an intermediate rate of transparency and another intermediate rate of reflection for other wavelengths.
{circle around (1)} Masahiro Ogusu, Tazuko Tomioka, Shigeru Ohshima, xe2x80x9cReceptacle Type Bi-directional WDM Module Ixe2x80x9d, Proceeding of the 1996 Electronics Society Conference of IEICE, C-208, p208 (1996).
The module proposed by {circle around (1)} has light paths in a free space. A PD and an LD form an independent PD module and an independent LD module which are separated in the free space. Spatial separation decreases crosstalk between the PD and the LD, which is an advantage. Since separated modules are integrated in the free space, the module is bulky, large, heavy and expensive.
Another known module divides an optical path by a Y branch coupler. The y-branch module is made by fabricating an inverse y-shaped branch lightwaveguide on a silicon bench, positioning a photodiode at an end of the lightwaveguide, providing a wavelength division multiplexer (WDM) at a branch point, putting a laser diode (LD) at an end of a right branched waveguide, and putting an end of an optical fiber at another end of a left branched lightwaveguide. The optical fiber emits 1.55 xcexcm receiving signals to the left branched lightwaveguide. The 1.55 xcexcm receiving signals make a straight way via the WDM in the lightwaveguide into the PD. The LD emits 1.3 xcexcm transmitting signals. The 1.3 xcexcm signals run in the lightwaveguide. The transmitting 1.3 xcexcm signals are reflected by the WDM and go into the optical fiber.
{circle around (2)} Japanese Patent Laying Open No.11-68705, xe2x80x9cTwo-way WDM Optical Transmission Reception Modulexe2x80x9d,
proposed such a Y-branched WDM based planar type module, in which receiving light going out from an optical fiber propagates in a lightwaveguide, passes the WDM and reaches a PD and transmitting light yielded by an LD is reflected by the WDM and enters the optical fiber. The LD and the PD are allocated at opposite ends of an silicon bench with a wide distance. {circle around (2)} asserted that such a structure decreases electric crosstalk and optical crosstalk.
The PD and the LD are mounted upon the common silicon bench. Silicon, which is a semiconductor with high conductivity, conducts electricity. Thus, electric crosstalk would be large. Silicon is transparent for 1.3 xcexcm light. Thus, optical crosstalk would be large in the silicon bench based module. The planar Y-branch type module has another drawback that the LD and the PD mounted on the surface require wide areas on a silicon bench. It is difficult to modify the planar single LD/PD module into a multichannel module having a plurality of pairs of LDs and PDs.
A third type known module is an upward branch type which is prepared by making a linear lightwaveguide/linear optical fiber on a planar bench, disposing a laser diode (LD) at an end of the lightwaveguide/fiber, providing a upward slanting WDM in the lightwaveguide/fiber and positioning a photodiode on a submount put at a point slanting to the WDM on the bench. Receiving light propagating in an external fiber and going into the lightwaveguide is reflected by the WDM upward to the PD.
{circle around (3)} T. Uno, T. Nishikawa, M. Mitsuda, G. Tohmon, Y. Matsui, xe2x80x9cHybridly Integrated LD/PD Module with Passive-alignment Technologyxe2x80x9d, Proceeding of the 1997 Electronics Society Conference of IEICE, C-3-89, p 198 (1997).
The above proposed such an upward branch type which was made by preparing a silicon bench having a lower front step and a higher rear step with a V-groove, gluing a glass substrate having a V-groove on the front step of the silicon bench with the V-groove aligning the bench V-groove, mounting an optical fiber on the V-grooves, installing a WDM in a slit at an intermediate spot of the fiber, mounting a photodiode on a submount at a point slantingly in front of the WDM, and fitting a laser diode at the end of the fiber. Receiving light running in the fiber is reflected upward by the WDM and is guided into the PD. Light passage is vertically divided for the LD and the PD.
Since the PD should be mounted slightly higher than the LD, the PD is mounted on the submount giving an additional height. An interval between the WDM and the PD is very short. Light path length between the WDM and the PD is small. The height difference of the PD and the LD is as small as a diameter of the optical fiber. Then, the PD and the LD are laid nearly on the same height.
{circle around (4)} Japanese Patent Laying Open No.11-218651, xe2x80x9cOptical Transmission and Reception Modulexe2x80x9d,
proposed a module having a transmitting part separated vertically from a receiving part, which had been invented by the same inventor as the present invention. A ground metallize is sandwiched between the separated transmitting part and the receiving part. FIG. 13 denotes a vertical section of the proposed device for showing gluing parts. A first substrate 95 has a through-hole and a bottom ground plane G. The first substrate 95, which is allotted to be the transmitting part, has a lightwaveguide, a WDM 97 on the top surface and an LD 98. A second substrate 99 has a through-hole and the ground plane G on the top. The second substrate 99, which is allocated to be the receiving part, has a PD 102 at the bottom of the holes and an AMP 103 near the PD 102. The first substrate is glued to the second substrate at the ground plane G. An end of an optical fiber 105 is joined to an end of a lightwaveguide 96.
Transmitting light signals emitted from the LD go into the optical fiber via the WDM. Receiving light signals propagating in the optical fiber are reflected by the WDM downward, pass the holes and go into the PD. The intermediate ground metallize G is commonly connected both to the LD part and the PD part. The ground metallize G, which is just on the plane joining the PD part to the LD part, prevents LD electromagnetic noise from invading to the PD. The WDM separates the optical path into an upper path and a lower path. The PD is mounted on a spot distanced from the first substrate having the lightwaveguide. The intermediate ground metallize G inhibits electromagnetic noise from the LD from inducing noise in the PD. This was a sophisticated LD/PD module proposed by the same inventor as the present invention.
The inventors had invented the vertical type LD/PD module as an ONU module. Thus, the module contains only a single LD, a single PD, a single waveguide and a single fiber. The ONU module has no need for mounting a plurality of pairs of PDs and LDs. The ONU module need not save a space or a volume for mounting plural LDs and PDs. {circle around (4)} turned out to have weak points yet. The intermediate ground metallize G, which was a gist of improvement of the module, has a tendency of transmitting LD noise to the PD by playing a role of antenna catching the LD electromagnetic noise and by fluctuating the ground level. The silicon substrates of the LD part and the PD part are coupled on the intermediate ground metallize G. A thin SiO2 film acts as a capacitor which allows AC electric currents to pass therethrough. The silicon substrates conduct electric currents, which induce electric noise in the PD part. Silicon is transparent for the nearinfrared light of the LD. Thus, the proposed {circle around (4)} was still incompetent to reduce crosstalk due to electric, electromagnetic and optical LD noise invasion to the PD via the silicon substrates and the ground metallize G.
There has been no requirement of multichannel LD/PD modules which contain a plurality of pairs of LDs and PDs so far. Many proposals have aimed at improvements of single-pair LD/PD modules containing only a single pair of LD and PD. Such a single-pair module does not invite a strong demand of reducing a unit size per an LD/PD pair. However, the multichannel LD/PD modules having a plurality of LD/PD pairs will be ardently required in near future. Unlike single-pair modules, reduction of a size per an LD/PD pair will be one of the most important problems for the multichannel LD/PD modules for preparing inexpensive, small-sized LD/PD modules.
Why does the new demand for the multichannel LD/PD modules occur? Bidirectional simultaneous communications send signals in an optical fiber in both directions simultaneously. Optical fibers join a plurality of ONUs (optical network units) to a single central station. N denotes the number of the ONUs. For communicating N ONUs, N optical fibers should be laid between the station and N ONUs. The present invention relates to an improvement of the bidirectional simultaneous optical communications. There are some alternatives for joining fibers.
At an early stage of building the optical communications, a 1:16 joint had been proposed. Sixteen ONUs (subscribers) are unified to a set. One main optical fiber is laid from the station for every set of the sixteen ONUs. The main optical fiber is divided into the sixteen ONUs by a 1:16 branch coupler which is laid near the sixteen ONUs. FIG. 14 shows the 1:16 joint system. The system is capable of reducing the number of fibers to N/16. The number of LD/PD modules installed in the station is N/16. The 1:16 joint has an advantage of sparing fibers and station modules. However, this system requires controlling the 1:16 branch coupler from the station and the ONUs. The additional controlling invites complexity of the 1:16 joint system and decreases flexibility for a change of ONUs.
At present, a simple 1:1 system (FIG. 15), which connects an ONU to a central station with a fiber without a branch coupler, has been examined instead of the 1:16 system. N optical fibers are laid between the single station and N ONUs. Although the 1:1 system requires longer fibers than the 1:16 system, the simple 1:1 system has an advantage of simple controlling and rich flexibility. However, the 1:1 system has another drawback of dilating required spaces and volumes for installing plenty of LD/PD modules at a central station in addition to a vast use of fibers.
In the 1:1 system, if the station employs LD/PD devices containing a plurality of LD/PD pairs in a unit, the number and the total volume of the units can be reduced. Employment of four LD/PD pair modules, eight LD/PD pair modules, sixteen LD/PD pair modules, . . . will reduce the number of the modules down to N/4, N/8, N/16, . . . at the station. For the reason, multichannel LD/PD modules are newly required as station modules.
One purpose of the present invention is to provide a small-sized multichannel optical communication device which contains a plurality of LD/PD pairs without increment of an installing space or volume. Another purpose of the present invention is to provide an inexpensive multichannel LD/PD module which is suitable for reducing cost per channel. A further purpose of the present invention is to provide a low-crosstalk multichannel LD/PD module which suppresses electromagnetic noise, electric noise and optical noise from transmitting from LDs to PDs. A further purpose of the present invention is to provide a small-sized multichannel optical communication device which contains a plurality of pairs of LDs yielding transmitting signals and PDs monitoring the LDs.
The present invention proposes a two-story module having a first story in a lower insulating case and a second story in an upper insulating case, allocating either story for an LD set having an optical connector, a silicon bench with lightwaveguides, a WDM(wavelength division multiplexer), LDs and an LD leadframe with leadpins, and the other story for a PD set having PDs and a PD leadframe with leadpins, filling the both stories with a transparent resin, and moulding the both cases with another hard resin. Every pair of an LD and a PD is contained in an imaginary plane vertical to a case base plane (case surface). The present invention features a vertical allotment of an LD and a partner PD, which alleviates a volume per LD/PD pair. Transmitting light beams emitted from the LDs propagate in the lightwaveguides on the silicon bench, pass the WDM without loss, go into optical fibers and propagate in the fibers to a counterpart node (a station or an ONU). Receiving light beams emitted from the optical fibers go into the lightwaveguides, propagate in the lightwaveguides, are reflected upward or downward by the WDM, fly slantingly in a resin-filled space, pass floor holes perforated on a bottom of the upper case, arrive at the PDs and make photocurrents carrying receiving signals. The PDs are shielded from the LDs by a bottom floor of the upper insulating, opaque case. The opaque upper case enables the module to suppress optical crosstalk between the LDs and the PDs.
Instead of a silicon case, an insulating case separates the PD part from the LD part, which reduces electric crosstalk via a case. An LD wiring part is entirely separated from a PD wiring part in the package, which reduces electrical crosstalk via wirings. Besides, no ground plate, which causes electromagnetic interference by fluctuation of a ground voltage, is interposed between the LD part and the PD part. Elimination of the ground metallize reduces electromagnetic crosstalk between the LD part and the PD part.
LDs or PDs are exclusively allotted to either the upper case (second floor) or the lower case (first floor). Allocation of PDs or LDs to the first floor or the second floor is optional. In both cases, the whole of the upper case and the lower case is molded into a body with a hard resin. The present invention allots the LD part including the lightwaveguides and the PD part separately to either the upper case or the lower case for guiding light signals in a vertical direction. A plurality sets of an LD, a lightwaveguide, a PD and an fiber can be arranged side by side in the horizontal direction vertical to the axial line, which alleviates the width and the volume of a device. The present invention is the most suitable for multichannel LD/PD modules.
This invention is applicable to a multichannel LD module having a plurality of pairs of LDs and monitoring PDs in addition to multichannel LD/PD modules. The teaching of the present invention gives a multichannel LD module by preparing an upper case and a lower case, allotting an optical connector, a lightwaveguide-formed silicon bench, a beamsplitter, LDs and LD leadpins to one of the two cases, allotting monitoring PDs and PD leadpins to the other of the two cases, unifying the lower case with the upper case, filling both cases with a transparent resin. LDs produce transmitting signal light beams. Parts of the LD produced light beams propagate in the lightwaveguides, pass the beamsplitter, and go into fibers on one floor. Other parts of the LD produced beams are reflected by the beamsplitter to the monitoring PDs which are mounted on the other floor. The PDs are not signal detecting PDs but LD power monitoring PDs.
PDs and LDs are installed in different floors. There are two probable versions. One is a set of a PD ground floor and an LD second floor. The other is a set of an LD ground floor and a PD second floor.
The PDs are not signal receiving PDs but monitoring PDs for checking a change of the power of LDs. Prevention of crosstalk is not a problem. M LDs and M PDs are allocated at vertically different positions. When it is difficult to mount monitoring PDs just at the back of LDs, the vertical allocation of LDs and PDs is useful. For example, it is sometimes desired that LD-driving ICs should be installed just behind the LDs. In usual, LD-driving ICs are provided in external circuits and the LDs and the LD-driving ICs are connected via leadpatterns, leadpins and Au wires. Long wirings between the LDs and the ICs distort signals through large inductance L of the wirings. The higher frequency the signal has, the more the signal distorts. High speed optical communications requires the LD modules to lay LD-driving ICs just behind the LDs for reducing signal distortion by shortening wirings. In the case, the LD-driving ICs are obstacles for placing monitoring PDs just behind the LDs. The present invention, which allocates LDs and monitoring PDs to different floors, is useful for making high speed LD modules having LDs connected with LD-driving ICs with short wires.
LD modules of a set of LDs and monitoring PDs take a similar structure to a set of LDs and signal-receiving PDs. Allocation of LDs or PDs to a first floor or a second floor is optional. It is preferable to fill unified cases with a transparent resin for decreasing random scattering and reflection.
The LD part with lightwaveguides and the monitoring PD part are separately allocated exclusively either to the upper floor or to the lower floor. The structure saves horizontal area. The present invention is suitable for multichannel LD modules since a plurality of LD/PD pairs can be arranged in parallel in a horizontal direction.
The present invention employs a two-story package with an upper case and a lower case and allocates either to a transmission portion and the other to a receiving portion. The transmission portion includes a fiber connector, a silicon bench with lightwaveguides, a WDM, LDs, LD leadpins and an LD wiring leadframe. The receiving portion includes PDs, AMPs, PD leadpins and a PD wiring leadframe (leadpins are a portion of the leadframe). The receiving light signals propagating in the fibers are reflected either upward or downward by the WDM. An inner space of the two-story package may be left to be an air-occupied space. Optionally, the inner space is filled with a transparent resin for reducing random scattering or reflection of light.
There are two types for allocating a PD part or an LD part to either a lower floor or an upper floor. One is an upper PD and a lower LD type and the other is an upper LD and a lower PD type.
[Upper PD/Lower LD Type]
A first story includes a transmitting part with LDs and a second story includes a receiving part. A lower case includes an optical connector having fibers and a silicon bench with M V-grooves, M lightwaveguides and M laser diodes (LDs) (Mxe2x89xa71). An upper case has M photodiodes (PDs) and optionally M preamplifiers (AMPs).
Allotment of higher PDs and lower LDs can be also applied to an LD module of two stories by displacing the signal detecting PDs by monitoring PDs. In the two story LD module, a lower case has a silicon bench with M lightwaveguides and M LDs, M fibers and an optical connector. An upper case has M monitoring PDs. Optionally auto-power controlling ICs closely accompany the monitoring PDs in the upper case.
[Upper LD/Lower PD Type]
A first story includes a receiving part with PDs and a second story includes a transmitting part with LDs. An upper case includes an optical connector having fibers and a silicon bench with M V-grooves, M lightwaveguides and M laser diodes (LDs) (Mxe2x89xa71). A lower case has M photodiodes (PDs) and optionally M preamplifiers (AMPs).
Allocation of lower PDs and higher LDs can be applied to an LD module of a two stories by displacing the signal detecting PDs by monitoring PDs. In the LD module case, the M monitoring PDs can be optionally accompanied by auto-power controlling ICs (APC-ICs) in the same lower case. The LD module has an upper case containing a silicon bench with M V-grooves and M lightwaveguides, M LDs, M fibers and an optical connector like the LD/PD module.
[Transparent Resin]
The upper case and the lower case are filled with a transparent resin without air gap for reducing reflection and random scattering at interfaces between the fibers and the space or between the lightwaveguides and the space. Requirements for the resin are transparency and a refractive index similar to the fiber (n=1.46). Similarity of the refractive indices reduces the reflection loss at the interface. For example, the resin is one of silicone resins or acrylate resins. In addition to the reduction of reflection loss, the resin has sufficient elasticity which protects PDs, LDs, AMPs and wires from external shock or force.
[Substrate]
An optimum material of a substrate for making lightwaveguides and LDs thereupon is silicon (as a silicon bench). Alternatives are ceramic substrates or polymer substrates.
[Case]
A upper case and a lower case can be made by insert-molding a resin and a leadframe in a metallic mould. The leadframe has leadpins and wiring patterns in a thin metallic plate. The insert-molding dispenses with the steps of printing metallized wiring patterns on an insulating substrate and sticking the insulating substrate into the cases. Liquid crystal polymer can be a resin of forming cases. Choice of liquid crystal polymer gives low-cost cases. Otherwise, ceramic cases are also available for the upper cases and the lower cases. In the case of the ceramic packages, metallized patterns should be made by printing, evaporating, sputtering and etching metals upon surfaces of the ceramic packages and leadpins should be brazed on peripheral wiring pads. Ceramic cases, which require higher cost than resin packages, excel in air-tightness, sealing, water-proof and life time. The following description relates mainly to metal-unified plastic cases insert-molded with leadframes.
[Lightwaveguide]
Lightwaveguides are made with quartz or polymers. Polymer waveguides, which can be easily fabricated upon resin substrates, can be low-cost lightwaveguides. In the case of silicon substrates, quartz waveguides are also available. A set of a silicon substrate and quartz lightwaveguides, which raises cost, has an advantage of low propagating loss.
[Number of LD/PD Pairs]
The present invention includes a single or a plurality of LD/PD units. xe2x80x9cMxe2x80x9d denotes the number of LD/PD units. An inequality Mxe2x89xa71 indicates the scope of the present invention. In any value of M, the present invention separates PDs from LDs by positioning LDs and PDs at different floors. The vertical allocation reduces the area occupied by LDs and PDs by arranging LD/PD effectively in a restricted space. The LD/PD modules of the present invention are preferable for multichannel optical communications devices. The present invention is also applicable to multichannel LD modules which contain a plurality of pairs of LDs and monitoring PDs. The LD module allocates LDs and monitoring PDs to different floors at different heights on a two-storied package.
An optical communications system connects a central station to a plurality of subscribers (ONU; optical network units) with optical fibers. xcex1 (e.g., 1.3 xcexcm band) denotes a wavelength of upward signal light from ONUs to the central station. xcex2 (e.g., 1.55 xcexcm band) denotes a wavelength of downward signal light from the central station to ONUs. xe2x80x9cNxe2x80x9d designates the number of ONUs. An ONU module, which should have a single transmitting device and a single receiving device, is a single pair module of M=1. At an ONU, upward xcex1 is the transmitting signal light which should be generated by an LD and downward xcex2 is the receiving signal light which should be sensed by a PD. N ONUs require N single LD/PD modules.
At the central station, the relation of the wavelengths is reversed. Downward xcex2 are transmitting light signals which are produced by LDs at the station. Upward xcex1 are receiving light signals which are sent from ONUs and are detected by PDs at the station. Instead of the sixteen branch network, which is annoyed at a complex relay element-controlling system, as shown in FIG. 14, a non-branch 1:1 fiber network which connects a station to N-ONUs with N independent fibers is now a prevailing candidate (shown in FIG. 15). If the communication system employs the non-branch network, the station requires N LDs and N PDs for N ONUs. If the station takes a single channel LD/PD module (M=1), N LD/PD modules should be equipped at the station, which occupies a huge volume in the station. If the station adopts four channel LD/PD modules (M=4), N/4 LD/PD modules are enough for the station.
Furthermore, use of eight channel LD/PD modules (M=8) enables the station to reduce the number of modules to N/8. N/16 sixteen channel LD/PD modules satisfy the requirement of the station. The reduction of the number of the modules is favorable for the central station having a poor extra space for storing the modules. The consideration clarifies that multichannel modules are preferable for the station.
Many proposals have been suggested for single-channel LD/PD modules. But, little multichannel devices have been suggested hitherto. Multichannel LD/PD modules which dispense with a wide space will be strongly required in future. The present invention is preferable for the requirement of multichannel devices.
[Leadframe]
Both the upper case and the lower case are resin cases transfermolded with leadframes. The PDs are not laid upon the silicon bench but upon the leadframes. The receiving signal light beams, which are reflected slantingly by the WDM midway on the waveguide, pass bottom holes and enter counterpart photodiodes. The bottom holes are not perforated on the silicon bench but on the metallic thin leadframe. The bottom holes, which are made at a stroke with other wiring parts by punching thin metal plates, require no extra step of perforating. The cited references {circle around (4)} (Japanese Patent Laying Open No. 11-218651) includes the step of mechanically drilling penetrating holes on a silicon bench. It takes long time to drill holes on a rigid silicon bench. This invention, which needs not perforated holes on a silicon bench, is superior to {circle around (4)} in perforating holes.
[Optical Crosstalk]
This invention enables the LD/PD device chips to reduce a volume per a unit LD/PD. In addition to the reduction of the volume per a unit, the present invention excels in alleviating optical crosstalk and electrical crosstalk between PDs and LDs. The two-story structure of the package allows the present invention to allocate LDs to a first floor or a second floor and to allocate PDs to the other floor. A thick bottom floor separates PDs from LDs. The bottom plate of the upper case suppresses optical crosstalk by shielding the PDs from the light emitted from the LDs. Silicon, which has a narrow band gap, allows light of a wavelength from 1 xcexcm to 1.6 xcexcm to pass through. Inherently, Si-benches, which are transparent to the near-infrared wavelengths (1 xcexcm to 1.6 xcexcm), are impotent for prohibiting optical crosstalk. The present invention shields noise light from LDs by the leadframes and the opaque resin (e.g., epoxy).
[Electrical Crosstalk]
The present invention separates PDs from LDs in a vertical direction as well as in horizontal directions. An insulating resin package intervenes between LDs and PDs. The related reference {circle around (4)}(Japanese Patent Laying Open No.11-218651) separates PDs from LDs with silicon bench. Silicon leads electricity. {circle around (4)} is annoyed at large electrical crosstalk via the silicon bench. The present invention, which separates PDs from LDs by a resin package instead of silicon, succeeds in suppressing electrical crosstalk.
[Electromagnetic Crosstalk]
The PDs are distanced in both vertical and horizontal directions from the LDs which are strong sources of electric, electromagnetic and optical noise for the PDs. The grounds and wirings of the PDs are all separated from the grounds and wirings of the LDs by the cases. Separation of the wirings and grounds reduces electromagnetic crosstalk.
The prior reference {circle around (4)} (Japanese Patent Laying Open No.11-218651) proposed an LD/PD module which was made by mounting an LD upon a first silicon bench, forming a ground metallize on a bottom of the first silicon bench, mounting a PD upon a second silicon bench, gluing the bottom of the second PD silicon bench to the bottom of the ground metallized bottom of the first LD silicon bench.
The intermediate ground (earth terminal) was a common ground both for the LD and the PD. The ground metallize would inhibit electromagnetic waves from flying from the LD to the PD.
The fact was otherwise. The thin metallized common ground, which was connected to an external ground via big resistance, was not a rigid ground. The thin metallize acts as an antenna, which catches electromagnetic waves from the LD, instead of the ground. The level of the metallize fluctuates by a varying LD level. Signal levels of the PDs and the AMPs are also varied by the fluctuation of the ground level. The wide, thin intermediate ground metallize turned out to be not effective but harmful for reducing electromagnetic crosstalk.
The present invention excludes a wide intermediate common ground between the PD part and the LD part. Since the effective antenna is eliminated, the PDs are insensitive to electromagnetic noise from the LDs. The wirings of the LDs are independent of and separated from the wirings of the PDs, because two leadframes are allocated exclusively to the PDs or the LDs in the upper or lower cases.
The present invention proposes optical communication modules of two-storied packages which allocate PDs and LDs to different stories. The PDs are separated from the LDs in a vertical direction in addition to horizontal directions. Vertical separation of PDs from LDs reduces electric crosstalk, electromagnetic crosstalk and optical crosstalk. Vertical allotment of PDs and LDs enables this invention to reduce chip occupation areas in comparison to the conventional modules having PDs and LDs on the same plane in a case. The effect of size-reduction is conspicuous, in particular, in the case of containing a plurality of sets of PDs and LDs (M=4, 8, 16 . . . ). The present invention is promising as a multichannel optical communication tool.
When a central stem station is connected to N ONUs (optical network units; subscribers) with N optical fibers without branch, a simple LD/PD module having a single pair of PD and LD is sufficient for each ONU. The stem station would require N LD/PD modules, if the station makes use of single pair LD/PD modules. N modules would take a vast volume for storing the modules in the stem station, which would enhance the cost of optical communications.
Use of a module of four pairs of LDs and PDs of the present invention alleviates the number of the modules installed in the station down to one fourth (N/4) of the number N of ONUs. Furthermore, employment of sixteen pair LD/PD modules of the present invention can reduce the number of LD/PD modules to one sixteenth (N/16) of N at the station. The present invention allows the stations to alleviate the space for modules by decreasing the number of modules. The present invention is suitable for the LD/PD modules at stations.
The relation of the transmitting/receiving signals xcex1 and xcex2 is inverse for the station module and the ONU modules. An optical communication system sends xcex2 (1.55 xcexcm band) signals from a central station to ONUs and sends xcex1 (1.3 xcexcm band) signals from the ONUs to the station. ONU modules have an LD for making xcex1 transmitting signals and a PD for detecting xcex2 receiving signals.
In this case, the multichannel module at the station should have LDs producing xcex2 (1.55 xcexcm band) transmitting signals and PDs for catching xcex1(1.3 xcexcm band) signals. A WDM at the station has wavelength selectivity of allowing xcex2 to pass and reflecting xcex1. FIG. 1 and FIG. 7 show such a case.
The WDM filter plays a wavelength-selective role of reflecting all the receiving beams from the optical fibers toward the upper case PDs or the lower case PDs and allowing all the transmitting beams from the LDs to pass therethrough.
Station LD/PD modules should have four, eight, sixteen pairs of LDs and PDs (M=4, 8, 16 . . . ) for reducing the space for storing the LD/PD modules. When LDs and waveguides are arranged with a 0.25 mm pitch on a silicon bench, sixteen LDs, for example, require a width of 0.25 mmxc3x9716=4 mm. An increase of required width in comparison to a single pair module is small for a sixteen-pair module or an eight-pair module. This invention gives small sized multichannel LD/PD modules (M=4, 8, 16, 32, 64 . . . ) having the same size or a similar size of packages.
Of course this invention can be applied to ONU modules having a single pair of LD and PD (Embodiment 3). An ONU LD/PD module should have xcex1 (1.3 xcexcm band) transmitting signals made by an LD and xcex2 (1.55 xcexcm band) receiving signals for being caught by a PD. An ONU WDM filter should allow xcex1 to pass and reflect xcex2.
The present invention can be applied to an LD module without signal detecting PDs. Laser diodes (LDs) degrade year by year. Laser power attenuates by the degradation. Preferably photodiodes should be provided in the vicinity of the LDs for monitoring the output power of the laser diodes. In the case of multichannel LD modules, it is difficult to arrange PDs just behind the LDs. Sometimes LD-driving ICs are installed just behind the object LDs, because long wiring distances between the LD-driving ICs and the LDs would distort the LD signal shapes.
The present invention allocates a set of LDs and another set of monitoring PDs exclusively either to the first floor or to the second floor for guiding parts of the LD beams reflected by the beamsplitter into the monitoring PDs. The beamsplitter, which has no wavelength selectivity, reflects parts of the LD transmitting beams and introduces the rest of the beams to the monitoring PDs on the lower floor or on the upper floor. The monitoring PDs detect the power of the LDs. The photocurrents of the monitoring PDs are input to the APCICs for controlling the power of the LDs at a constant level. There have been many LD modules having an LD which emits forward light and backward light, an optical fiber which sends the forward emitted LD light as transmitting signals, and a PD which detects the backward emitted LD light behind the LD. There has been no LD module in which a monitoring PD detects forward LD light. In the case of multichannel LD modules, which have poor margins behind LDs, vertical allocation of LDs and PDs of the present invention is advantageous.